As the use of new renewable energy is rapidly increasing, a demand for an energy storage device using a battery is rapidly increasing. Among such batteries, a lead battery, a nickel/hydrogen battery, a vanadium battery and a lithium battery may be used. However, the lead battery and the nickel/hydrogen battery have a very low energy density, and need a larger space for storing the same amount of energy. In addition, the vanadium battery has the problems of environmental pollution due to the use of a heavy metal-containing solution, and performance degradation due to the immigration of a small amount of a material between an anode and a cathode through a membrane separating the anode and the cathode, and thus, has not been commercialized on a large scale. The lithium battery having excellent energy density and output characteristic is technologically advantageous, but uneconomical to be used as a secondary battery for a large-scale power storage due to a resource scarcity of a lithium material.
In order to solve the foregoing problems, many attempts have been made to use sodium which is an abundant resource in earth as a material of the secondary battery. Among them, as disclosed in U.S. Patent Application Publication No. 20030054255, a sodium-sulfur battery using beta alumina having a selective conductivity for sodium ions, and having an anode impregnated with sodium, and a cathode impregnated with sulfur, is currently being used as a large-scale power storage device.
However, as to the existing sodium-based secondary battery such as the sodium-sulfur battery or a sodium-nickel chloride battery, considering the conductivity and melting points of the battery components, the sodium-nickel chloride battery should have an operating temperature of 250° C. or more, and the sodium-sulfur battery should have an operating temperature of 300° C. or more. Due to such problems, those batteries are disadvantageous in economic aspect in manufacture or operation, since temperature maintenance, sealability maintenance and safety of the batteries should be reinforced. In order to solve the foregoing problems, a room temperature-type, sodium-based battery is being developed, however, which has a very small output, and much less competent than a nickel-hydrogen battery or a lithium battery.